What are the functions of white blood cells (leukocytes)?

Functions of White Blood Cells (Leukocytes)

White blood cells serve as the body's primary defense system, protecting against invading pathogens through innate and adaptive immunity, while also participating in wound healing, tissue repair, immune regulation, and tumor surveillance. 1, 2, 3

Primary Defense Functions

Pathogen Elimination and Innate Immunity

• Neutrophils (polymorphonuclear leukocytes) constitute the "first line of defense" against invading microorganisms, accumulating rapidly at infection sites to neutralize pathogens through direct microbicidal activity 2, 3
• Granulocytes, which account for approximately 66% of total WBCs, respond most strongly to acute threats as part of the fast innate immune system's fight-or-flight reaction 4
• Monocytes and macrophages participate in chronic inflammation by synthesizing numerous mediators and eliminating various pathogens through phagocytosis 2

Adaptive Immune Response

• Lymphocytes orchestrate antigen-specific immunity through three major mechanisms: secreting cytokines for immune signaling, directly killing virus-infected and tumor cells, and producing antibodies 2, 5
• T cells, B cells, and natural killer cells each have distinct roles in antibody production, cell-mediated killing, and immune response regulation 5
• Lymphocytes are transported via lymphatic vessels to lymph nodes where they stimulate antigen-specific immune responses 6

Tissue Protection and Repair Functions

Inflammation Regulation

• Leukocytes occupy a pivotal position in both initiating and resolving inflammation, with their activation driven primarily by stress hormones including catecholamines and cortisol 4, 3
• In human milk, leukocytes protect both the infant and mammary gland against infections through direct and indirect mechanisms, while promoting development of immunocompetence 4
• The inflammatory response must be carefully balanced—inadequate responses risk overwhelming sepsis, while excessive responses lead to systemic inflammation and multiple organ damage 3

Wound Healing and Tissue Remodeling

• Leukocytes play fundamental roles in wound healing and tissue remodeling beyond their antimicrobial functions 3
• These cells participate in chronic inflammation processes necessary for tissue repair, though prolonged activation can contribute to pathological conditions 2

Specialized Leukocyte Populations

Differential Cell Type Functions

• Neutrophils increase for 4-6 hours after cessation of exercise, while lymphocytes show a biphasic pattern with initial increase followed by prolonged decrease up to 36 hours 4
• Monocytes (4-10% of WBCs) increase after various types of exercise and typically return to baseline within 2 hours 4, 7
• Natural killer cells demonstrate particularly strong responses to acute stress, increasing 16-45% immediately after exercise 4

Immune Surveillance

• Leukocytes provide tumor surveillance, though they are notably less efficient at protecting against cancer ("the enemy from within") compared to infectious threats 6
• Circulating leukocytes represent less than 5% of the body's total leukocyte pool, meaning blood counts do not necessarily reflect tissue-specific immunological reactions 8

Clinical Significance

Dynamic Response Patterns

• Leukocyte concentrations increase when either the mother or infant have an infection, demonstrating their responsive nature to pathological conditions 4
• The increase in WBCs during acute exercise is a protective mechanism that preconditions the body for contact with potential penetrating pathogens 4
• Different WBC subsets respond differentially to stimuli, with granulocytes, lymphocytes, and monocytes each following distinct temporal patterns 4

Pathological Implications

• While leukocytes are essential for defense, they can contribute to organ damage in ischemia/reperfusion injury, trauma, sepsis, and organ transplantation through excessive free radical release and microvascular plugging 2, 3
• Cancer can exploit inflammatory processes driven by leukocytes to promote tumor progression and angiogenesis 6

Important Caveats

• Leukocytes circulate in a quiescent state and require rapid activation by pathogens or other stimuli—permanent activation would be unsustainable due to widespread microvascular plugging and excessive metabolic demand 3
• The ambiguous role of leukocytes in normal versus inflamed tissue remains incompletely understood despite extensive research 2
• Abnormal changes in leukocyte number and function serve as clinical indicators for infection, inflammation, cancer, leukemia, and coronary artery disease, making their analysis invaluable for diagnosis and monitoring 1